docs: def total_review_average(obj, normalize_to=100):
      """Returns the average for all reviews of the given object."""
      ctype = ContentType.objects.get_for_model(obj)
      total_average = 0
      reviews = models.Review.objects.filter(
          content_type=ctype, object_id=obj.id)
      for review in reviews:
          total_average += review.get_average_rating(normalize_to)
      if reviews:
          total_average /= reviews.count()
      return total_average